Hard Loopback testing

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Mar 24th, 2008
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Hi There;

I have a WIC-2T serial card and a smart serial/DB25 Cisco lead (CAB-SS-530MT). I need to test the cable using a loop back. Does anyone know the pinouts that need to be crossed to perform this test?


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dgahm Mon, 03/24/2008 - 18:53
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Here is the pinout:


Do you have a CSU/DSU you can plug it into to obtain a loopback (local loop)? This would be the easiest way to test the cable and router interface.

EIA-530 uses a balanced pair for tx and rx, and is also synchronous, meaning the DTE side, in this case the router, needs clock signals. It may be possible to use the ext clock from the router to drive TX clock and RX clock, but I have never tried it.

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austindaz Mon, 03/24/2008 - 19:28
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Hi Dave;

How do you mean - 'use the external clock from the router?' Correct me if I'm wrong but what you're saying is that because its DTE, despite having the correct loopback, I still need a clock source to make it happen?


dgahm Tue, 03/25/2008 - 08:49
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Yes, normally the router, if using a DTE cable, needs to obtain clocking from the DCE. Sometimes a tail circuit needs to clock a DCE which is where the external clock comes in (TXCE+ TXCE-). If the router drives those clock signals (not sure, never tested it), you could connect that to the TXC and RXC inputs. So you would have to loop the TX and RX pairs and connect TXCE to TXC and RXC. Try it and see what happens. Do you have a breakout box?



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